1. Field of the Invention
The present invention relates to a light emitting display device, and more particularly, to a light emitting display device that is capable of avoiding a brightness difference between respective pixels resulting from a voltage variation, and a method for driving the same.
2. Discussion of the Related Art
Recently, various flat panel display devices have been developed to reduce weight and volume which are disadvantages of a cathode ray tube. These flat panel display devices may be, for example, a liquid crystal display, a field emission display, a plasma display panel, a light emitting display, and the like.
The light emitting display, among the flat panel display devices, is of a spontaneous emission type wherein fluorescent material is excited due to recombination of electrons and holes to emit light. Such light emitting displays are roughly classified into an inorganic light emitting display device that employs an inorganic compound as fluorescent material and an organic light emitting display device that employs an organic compound as fluorescent material. These light emitting displays are expected to replace the cathode ray tube displays owing to their many advantages, such as, low-voltage driving, self-luminescence, thinness, wide viewing angle, high response speed, high contrast, etc.
An organic light emitting element generally has an electron injection layer, electron transport layer, light emitting layer, hole transport layer and hole injection layer interposed between a cathode and an anode. In a light emitting display device using this organic light emitting element, when a certain voltage is applied between the anode and the cathode, electrons generated from the cathode move to the light emitting layer through the electron injection layer and electron transport layer, and holes generated from the anode are moved to the light emitting layer through the hole injection layer and hole transport layer. As a result, in the light emitting layer, the electrons from the electron transport layer and the holes from the hole transport layer are recombined, thus emitting light.
A pixel of the light emitting display device generally has a light emitting element for emitting light in response to a drive current applied thereto, and a pixel circuit for operating the light emitting element. The pixel circuit includes first and second thin film transistors (TFTs) interconnected in the form of a current mirror. The first and second TFTs are supplied with a voltage from a single voltage source.
The first TFT conducts drive current corresponding to gray-scale current applied to a data line and supplies it to the light emitting element. In the light emitting display device, conventionally, gray-scale current larger than that corresponding to an image to be currently expressed is applied to the data line for the purpose of increasing the charging speed of the data line. This operation can be carried out on the premise that the mirror ratio between the first TFT and the second TFT must be set to a large value. That is, the channel width of the first TFT must be set to a smaller value and the channel width of the second TFT must be set to a larger value. This enables drive current flowing through the first TFT to have the value of gray-scale current corresponding to an image to be currently expressed.
When this mirror ratio is larger, larger gray-scale current can be applied to the data line. However, the mirror ratio is greatly restricted by a TFT design constraints. For this reason, it is not possible to unconditionally make the mirror ratio large. As a result, increasing the charging speed of the data line is still hampered by a big restriction.
In order to solve this problem, a technique has been proposed that is capable of applying different voltages respectively to the first TFT and second TFT to increase the difference between current flowing through the first TFT and current flowing through the second TFT without making the mirror ratio large.
A detailed description will hereinafter be given of a conventional light emitting display device based on the above technique.
FIG. 1 is a circuit diagram showing the structure of two pixels in the conventional light emitting display device.
The conventional light emitting display device comprises a display unit (not shown) that has a plurality of pixels defined by a plurality of gate lines GL and a plurality of data lines DL crossing each other substantially perpendicularly, as shown in FIG. 1.
Each pixel includes a first voltage line VL1 for supplying a first voltage VDD1, a second voltage line VL2 for supplying a second voltage VDD2, a pixel circuit 11 connected to the associated data line DL and gate line GL, and a light emitting element OLED connected between the pixel circuit 11 and a third voltage line VL3 that supplies a third voltage GND.
The pixel circuit 11 of each pixel includes first and second TFTs Tr11 and Tr12 interconnected via a node n for forming a current mirror, a capacitor Cst connected between the gate electrode and source electrode of the first TFT Tr11, a third TFT Tr13 for operating the second TFT Tr12 in a diode manner in response to a scan pulse from the gate line GL, and a fourth TFT Tr14 for forming a current path between the second voltage line VL2 and the data line DL in response to the scan pulse from the gate line GL. The first voltage line VL1 is connected to the first TFT Tr11 to supply the first voltage VDD1 to the first TFT Tr11. The second voltage line VL2 is connected to the second TFT Tr12 to supply the second voltage VDD2 to the second TFT Tr12.
Here, by setting the second voltage VDD2 to a higher value than the first voltage VDD1, it is possible to set gray-scale current flowing through the second TFT Tr12 to a larger value than drive current flowing through the first TFT Tr11 without increasing a mirror ratio between the first TFT Tr11 and the second TFT Tr12. The gray-scale current is sunk to a data driver (not shown) through a current path consisting of the second voltage line VL2, second TFT Tr12, fourth TFT Tr14 and data line DL.
However, the light emitting display device with this structure has the advantage of increasing the difference between the amount of current flowing through the first TFT Tr11 and the amount of current flowing through the second TFT Tr12 without increasing the mirror ratio, as mentioned above, but has the following problem because the first voltage line VL1 and the second voltage line VL2, independent of each other, are used.
That is, the first voltage line VL1 and the second voltage line VL2 are arranged in parallel with the data line DL. Each pixel arranged along the data line DL is connected in parallel to the first and second voltage lines VL1 and VL2, so as to receive the first voltage VDD1 and the second voltage VDD2 in common. Notably, as the light emitting display device becomes larger in size, the first and second voltage lines VL1 and VL2 are thus increased in length, thereby causing the first and second voltage lines VL1 and VL2 to have a larger amount of resistance and capacitance components. This becomes more serious toward the ends of the first and second voltage lines VL1 and VL2. As a result, brightness nonuniformity appears between the pixels connected in common to the first and second voltage lines VL1 and VL2. The reason is that the levels of the first and second voltages VDD1 and VDD2 from the first and second voltage lines VL1 and VL2 become lower toward the ends of the corresponding lines due to the resistance and capacitance components of those lines.
In particular, the distortion of the first voltage VDD1 from the first voltage line VL1 becomes a big issue, because the first voltage VDD1 is related to drive current to be supplied to the light emitting element OLED. In addition, because the first voltage VDD1 from the first voltage line VL1 is lower than the second voltage VDD2, it is more easily influenced by the resistance and capacitance components. In contrast, the second voltage VDD2 from the second voltage line VL2 is influenced less by the resistance and capacitance components, so that the pixels receive the second voltage VDD2 of substantially the same level. When the first voltage VDD1 varies like this, the voltage at the source electrode of the first TFT Tr11 varies. At this time, because the voltage at the gate electrode of the first TFT Tr11 is fixed in level, the voltage between the gate electrode and source electrode of the first TFT Tr11 varies in the end. As a result, the value of the drive current flowing through the first TFT Tr11 varies, thereby causing the light emitting element OLED of each pixel to exhibit different brightness with respect to the same gray-scale current. In conclusion, the picture quality of the light emitting display device is degraded.